1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method especially suitable for application to semiconductor lasers and light emitting diodes or electron mobility devices using nitride III-V compound semiconductors such as GaN.
2. Description of the Related Art
Heretofore, semiconductor lasers and light emitting diodes have been developed, using nitride III-V compound semiconductors made of group III elements such as Al, Ga and In and group V elements including N, as represented by GaN, as their light emitting materials in the range extending from green or blue to ultraviolet. On the other hand, although continuous oscillation at room temperatures has been realized with semiconductor lasers, there is still the need for further improvement in crystallographic property of nitride III-V compound semiconductors toward their longer lifetime. For this purpose, a technique for improving crystallographic property of GaN such as ELOG-GaN (Epitaxially Laterally Overgrown GaN) is being used and actually performing effects. However, it is also desired to further improve the basis of the technique, namely, crystallographic property of nitride III-V compound semiconductors grown on crystalline substrates.
Usually used as a substrate crystal is sapphire or SiC. Nitride III-V compound semiconductor layers are grown on such a crystalline substrate after it is processed in a growth apparatus for sufficiently smoothing, polishing and cleaning its surface. In order to manufacture a desired semiconductor device, it is necessary to make various multi-layered structures having surfaces or interface structures which are flat in the atomic level. Therefore, also for substrates, surface cleaning methods and growth methods capable of maintaining or making their surfaces flat are being used.
There has also been proposed a method which improves crystalline property of nitride III-V compound semiconductor layers by slightly inclining a crystalline substrate surface such that layers be grown thereon in a step flow mode (Japanese Patent Laid-Open Publication No. hei 7-201745).
When using a GaAs substrate or InP substrate, a technique is being used, which first grows a buffer layer on a substrate having formed on its (001) surface a (111) B-oriented diffraction grating for the purpose of reflecting guided light in the waveguide. (Japanese Patent Laid-Open Publication No. hei 8-264901).
Also known are a method for improving crystalline property by interposing an AlN buffer layer (Japanese Patent Laid-Open Publication No. hei 2-229476) or a GaN or AlGaN buffer layer (Japanese Patent Laid Open Publication No. hei 4-297023) between a sapphire substrate and a nitride III-V compound semiconductor thereon; a method for improving by nitrifying a sapphire substrate surface (Japanese Patent Laid Open Publication No. hei 5-41541). Also known is a method which sequential stacks a plurality of buffer layers different in lattice constant to reduce differences in lattice constant between the substrate and the buffer layer and between the buffer layer and an epitaxial layer thereon (Japanese Patent Laid Open Publication No. hei 9-63962).
It is generally noted that “fluctuation” in crystalline orientation exists in nitride III-V compound semiconductor crystals grown on a sapphire or SiC substrate. That is, these nitride III-V compound semiconductor crystals are “mosaic crystals”. Fluctuation in crystalline orientation is roughly classified into two components, namely, “twist” mosaic components which fluctuate in a rotational direction around an axis vertical to the substrate surface, and “tilt” mosaic components which are fluctuation in axial orientation from a vertical direction. Magnitude of these fluctuations is noted to be normally 0.01 to 0.5°. Causes of fluctuation in crystalline orientation are considered to lie in a difference in crystalline structure between the substrate and a nitride III-V compound semiconductor layer grown thereon and a large difference in lattice constant between them. It is also considered to be another factor that, crystal orientation is not accurately carried over during growth from the sapphire substrate because of relatively weak atomic coupling between stable sapphire crystal and a nitride III-V compound semiconductor crystal as compared with those inside these crystals.
Due to such mosaic crystalline property, in semiconductor light emitting devices using nitride III-V compound semiconductors, improvement in device characteristics has been prevented, by degradation in emission efficiency, and there are problems such as short lifetime regarding reliability required for the device.